1. Field of the Invention
Embodiments of the present invention relate to vibration member driving circuits.
2. Description of the Related Art
In recent years, with improved resolutions of image pickup elements, dust adhered to an optical system in use have affected captured images in image pickup apparatuses as optical apparatuses. Particularly, the resolutions of image pickup elements to be used in video cameras and still cameras have significantly been improved. For that, when external dust and/or dust (powder) such as wear debris caused on an internal mechanical rubbing surface are adhered to an optical member such as an infrared cut filter and an optical low pass filter near an image pickup element, the powder may appear on captured images because of less blurred images on an image pickup element surface.
An image pickup unit of a copy machine or facsimile machine as an optical apparatus scans a line sensor or scans a document placed closer to a line sensor to read a flat document. In this case, when dust adheres to a light beam launching area, the dust appears on a scanned image. One piece of dust may appear as a line image which continues in a document feeding direction and deteriorates the quality of the image in a reading unit of a facsimile machine which scans and reads a document or a reading unit of a copy machine which reads, that is, skims a document during transport from an automatic document feeder.
Japanese Patent Laid-Open No. 2008-207170 proposes a dust removing device which may move such dust in a desirable direction by exciting free traveling waves to a vibration member including an optical member. FIG. 3 is a schematic diagram illustrating a configuration of a dust removing device disclosed in Japanese Patent Laid-Open No. 2008-207170. A vibration member 501 having an optical member 502 is provided on a light entrance side of an image pickup element 503. Piezoelectric elements 101a and 101b which are electro-mechanical energy conversion elements are placed at different positions in the direction where nodal lines of out-of-plane bending vibration are aligned in the vibration member 501. Alternating voltages having an equal frequency and a phase difference of 90° are applied to the piezoelectric elements 101a and 101b. 
The frequency of the alternating voltage to be applied is between a resonance frequency in an m-order (where m is a natural number) vibration mode which bends out of plane along the longitudinal direction of the vibration member 501 and a resonance frequency in an (m+1)-order vibration mode. In the vibration member 501, vibrations in the m-order vibration mode with a response with a resonance phenomenon and vibrations in the (m+1)-order vibration mode with a 90° time phase difference (phase advanced by 90° from the m-order out-of-plane bending vibration) are excited with an equal amplitude and at equal vibration periods. Then, in the vibration member 501, synthesized vibrations (free-traveling waves) are generated in which the vibrations in two vibration modes are synthesized. The synthesized vibrations move the dust on the surface of the vibration member 501 in a desirable direction.
However, in a dust removing device in the past, the amplitude change of alternating voltage to be applied to the piezoelectric element 101, that is, the gradient of a frequency characteristic of the alternating voltage may increase near the resonance frequency of the vibration member 501 in some frequency band to be used. For that reason, the alternating voltage largely changes when the resonance frequency of the vibration member 501 varies because of individual differences or the heat while the device is being driven changes the resonance frequency. The alternating voltage increased more than necessary may increase the power consumption because of a current increase or damage the optical member 501 because of the increase of the vibration amplitude excited in the vibration member 501. When the alternating voltage is lower than necessary voltage, a sufficient vibration amplitude of the out-of-plane bending vibration to be excited in the vibration member 501 may not be acquired, and the dust removal efficiency may decrease.